Photosensitive chromoproteins such as rhodopsin are characterized in that they can absorb visible rays and efficiently convert them into a chemical work through a cyclic reaction system. As the results of light-absorption, rhodopsins can actively transport a proton in one direction, which is called a proton pump. Well known examples of rhodopsin photosensitive chromoproteins include rhodopsin and bacteriorhodopsin which are visual pigments. In particular, the application of bacteriorhodopsin to photoelectric devices has attracted public attention, since it shows an excellent stability in vitro.
In order to harness the photoresponse of bacteriorhodopsin as a physical signal in vitro, photoelectrical conversion, which may be advantageously applied to devices, is commonly employed.
For the photoelectrical conversion, films comprising somewhat oriented molecules are generally required. These films are mainly produced by, for example, the electric field orientation method, static adsorption method and Langmuir-Blodgett (LB) method.
A commonly known method for photoelectrical conversion with the use of an oriented bacteriorhodopsin film comprises preparing a sandwich-type dry cell wherein said film is inserted between two electrically conductive electrode substrates and monitoring a photovoltaic response. This method is reported by, for example, K. Nagy (Biochem. Biophys. Res. Commun., 85, 383-390 (1978)) and G. Varo (Acta Biol. Acad. Sci. Hung., 32, 301-310 (1981)). In these examples, electrodeposited bacteriorhodopsin films produced by the electric field orientation method are used. This method is characterized in that a high photoelectromotive force (several V) can be obtained by using a relatively thick membrane (1 or above at a usual absorbance). Since the film has an extremely high resistance (usually 10.sup.10 M.OMEGA./cm), however, it is difficult in this method to obtain the response in the form of a photoelectric current which is superior from the viewpoint of linearity of the response dose being in correspondence with the absorbed light energy. An electric current response can be also obtained by, for example, converting an electric signal with the use of an electric field effect transistor (FET) as shown by JP-A-62-63823. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However the inherent nonlinearity of the output given by the generated photoelectromotive force cannot be improved thereby.
T. Furuno et al. (Thin Solid films, 160, 145-1151 (1988)) disclose a process for obtaining photoelectric conversion as an electric current signal which comprises preparing a sandwich cell by laminating a Langmuir-Blodgett (LB) film of a purple membrane containing bacteriorhodopsin onto an electrode. In this case, however, the obtained photoelectric current is extremely faint (in the order of 10.sup.-11 A), even though several tens of films are laminated.
These sandwich-type photovoltaic cells are further disadvantageous in that electrical leakage is frequently observed between two electrodes adhering photosensitive chromoprotein film(s). In the case of an ultra-thin film such as a LB film, in particular, it becomes more and more difficult to control the leakage as the thickness of the film decreases. The use of a LB film comprising a smaller number of layers is useless in practice, since the current output is lowered in this case. In the case of the above-mentioned dry cells, furthermore, the moisture contained in the films or in the environment would significantly affect the response sensitivity, which causes an essential problem in the reproducibility of the output.
Different from the system of these dry cells, it has been reported to employ a photosensitive chromoprotein film, which is prepared by using various carriers or lipid bilayer films, as a permeable membrane partitioning electrolyte and giving a difference in photopotential between the both sides of the permeable membrane between two electrodes in the electrolyte as a change in voltage or current (refer to, for example, K. Singh et al., Biophys. J., 31, 393-401 (1980); L. A. Drachev et al., FEBS Letters, 39, 43-43 (1974); M. C. Blok et al., FEBS Letters, 76, 45-50 (1977); JP-A-62-9228). However these methods are disadvantageous in that the photoresponsive film is not joined to the electrode material and thus the response is transmitted via the ion conduction of a solution, which causes an extremely low response rate (at a level of a second to a minute). Furthermore, the use of the permeable membrane makes it difficult to form a device in the form of a film.
Accordingly, JP-A-59-197849 and JP-A-62-11158 each propose a process wherein the phototransport conducted by bacteriorhodopsin in water is directly applied to a filmy base of a pH-sensitive transducer (in particular, an ion-sensitive FET=ISFET) to thereby give an electric signal. pH-sensitive or ion-sensitive electrodes including ISFET are characterized in that a change in proton or ion concentration is given as a change in the surface potential of an electrode material. That is to say, a so-called potentiometric means is employed in these systems. However, these potentiometric methods have some disadvantages, for example, a low accuracy, unstable data and a low response rate when applied to a transport protein such as rhodopsin.
Known photoelectric conversion systems with the use of photosensitive chromoproteins may be roughly classified into three types, namely, those wherein a film of said photosensitive chromoprotein is inserted between electrodes so as to harness the photoelectromotive force; those wherein said film is used as a permeable membrane in an electrochemical cell so as to harness the photoelectromotive force; and those wherein said film is fixed to an ion-sensitive transducer so as to potentiometrically detect a photoresponse. In the first type, the film should have a sufficient thickness in order to secure the output and to form a device. As a result, a large amount of the protein should be used, which causes a problem from an economical point of view. This type suffers from a further problem, namely, the response sensitivity is largely affected by, for example, moisture. In addition, in each of these three types the output is obtained as the electromotive force, and thus the response dose shows no linearity against the input, i.e., light amount. This fact causes another problem when they are to be applied to, for example, an optical sensor. The second and third types suffer from an additional problem, namely, a low response rate, when applied to, for example, an optical switch.
SUMMARY OF THE INVENTION
Accordingly, it is a first object of the present invention to provide a photoelectric transducer by using a photosensitive chromoprotein film which has a high sensitivity and shows a high response rate. It is a second object of the present invention to provide an amperometric photoelectric transducer which shows an excellent reproducibility of the output and gives a rapid response. It is a third object of the present invention to provide a photoelectric transducer which gives a photoresponse of a high output even though an extremely thin film corresponding to several layers of an LB film is used.
These objects of the present invention have been successfully achieved by a photoelectric transducer capable of affecting amperometric photoelectric conversion which comprises a photoresponsive electrode, which is provided with an oriented film of a photosensitive chromoprotein at the interface of an electrically conductive electrode substrate and an electrolyte, combined with a counter electrode.